Tubular container with a heat seal having non-symmetrical inner and outer beads

ABSTRACT

A sealed composite container for products is provided having a paperboard body ply with a liner ply adhered on the inner surface thereof. One end of the container is rolled outwardly to form a rim and exposing the inner surface of the liner ply, which comprises a heat sealable composition. A lid for the container, also having a layer of heat sealable composition, is placed adjacent to the exposed heat sealable composition of the container liner and heat sealed thereto. The heat seal has both an inner and an outer bead formed on either side of the heat sealed area, wherein the inner bead comprises a larger amount of the heat seal compositions than the outer bead. A method and apparatus for forming the heat seal is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/416,184,filed Oct. 11, 1999, now U.S. Pat. No. 6,264,098, which is incorporatedherein by reference, and which is in turn a continuation-in-part ofapplication Ser. No. 09/065,783, filed Apr. 23, 1998, now U.S. Pat. No.5,979,748, which is incorporated herein by reference, and which is inturn a continuation-in-part of application Ser. No. 08/796,793 filedFeb. 6, 1997, now U.S. Pat. No. 5,829,669, which is also incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to food containers and methods andapparatus for making food containers, and more particularly relates toheat seals used to seal such containers.

BACKGROUND OF THE INVENTION

Food and drink products and other perishable items are often packaged intubular containers, which are sealed at both ends. These tubularcontainers typically include at least one structural body ply and areformed by wrapping a continuous strip of body ply material around amandrel of a desired shape to create a tubular structure. The body plystrip may be spirally wound around the mandrel or passed through aseries of forming elements so as to be wrapped in a convolute shapearound the mandrel. At the downstream end of the mandrel, the tube iscut into discrete lengths and is then fitted with end closures to formthe container.

Tubular containers of this type typically include a liner ply on theinner surface of the paperboard body ply. The liner ply preventsliquids, such as juice, from leaking out of the container and alsoprevents liquids from entering the container and possibly contaminatingthe food product contained therein. Preferably, the liner ply is alsoresistant to the passage of gasses, such as oxygen and nitrogen, so asto prevent odors of the food product in the container from escaping andto prevent atmospheric air from entering the container and spoiling thefood product. Thus, the liner ply provides barrier properties and thebody ply provides structural properties.

In addition, current commercial containers often have membrane-type lidsor end closures heat sealed to a curled or bead-shaped rim of thecomposite container wall to form a peelable seal. The rim is formed byturning outwardly the end of the container to position the inner layerof the liner material on the outwardly curved surface.

A major difficulty in developing a usable heat seal between thecontainer lid and the rim of the container wall is balancing bondstrength with ease of opening for the end user. During transport, thesealed containers experience temperature and pressure extremes thatstress the heat seal and can lead to rupturing of the container. Thebond strength must be sufficient to withstand the rigors oftransportation. In particular, when containers packaged and sealed atone elevation are then subjected to lower ambient air pressure, such asduring air transportation or when transported to consumers at higherelevations, a relative positive pressure is created within the containerwhich could cause the seal between the lid and the container to rupture.This ability of the container to avoid rupturing under such conditionsis known as burst strength. However, as the burst strength increases,there is generally a concomitant increase in difficulty of opening ofthe container, which is exhibited by the peel strength or peelresistance of the container. The higher burst strength indiscriminatelyprevents both rupturing during transport and opening by the end user.

Certain types of heat sealable coatings have been used in both thelidding and liners of conventional containers. For example, SURLYN®polymer, a product of Dupont, is a material known in the art and iscommonly used as a heat seal coating. SURLYN® polymer is an ionicallycross-linked polymer with limited flow characteristics when heated.Typically, the layer of the container and the layer of the membranewhich contact each other are constructed of SURLYN® polymer, and may becoated with a wax. These two layers of SURLYN® polymer are heat sealedalong the top surface of the container bead. The two SURLYN®) polymerlayers create an extremely strong bond layer that remains relativelyuniform in thickness across the seal area. Due to the strongcross-linked bond created by SURLYN® polymer, however, opening thecontainer can require a peel force which is too high for some consumersand usually results in tearing and exposure of the other layers of thecontainer wall, such as the paperboard body wall, as is illustrated inU.S. Pat. No. 4,280,653 to Elias. This gives the top of the container aragged, undesirable appearance.

In the parent application, Ser. No. 09/065,783, the formation of twoheat seal beads is described. The two beads comprise an inner heat sealbead and an outer heat seal bead, each heat seal bead being formed ofthe heat sealable polymers of the seal layers of the membrane and theliner. The two beads are formed by using heat and pressure to force theheat sealable polymers to flow away from the central portion of the heatseal area and towards the interior and exterior of the container. Thereduction in the amount of heat seal material in the central heat sealarea reduces the bond strength in the central heat seal area and allowsopening of the container without unsightly tearing of the liner andexposure of the paperboard layer of the container wall. However,formation of the beads according to the parent application does notentirely erase the difficulty of balancing burst strength and ease ofopening.

It would be advantageous to provide a sealed container and method forsealing a container that combine improved ease of opening and anattractive appearance after opening with the seal strength and barrierproperties required for protection of the products within the container.

SUMMARY OF THE INVENTION

The composite container of the present invention successfully balancesthe need for ease of opening with the burst strength necessary tomaintain a hermetic seal despite changes in pressure routinelyexperienced during transportation of the container. The presentinvention provides a sealed container having a larger heat seal bead onthe interior side of the heat seal area than on the exterior side of theheat seal. The inner heat seal bead contains a greater amount of heatseal material and has a greater width than the outer heat seal bead.Since it had been discovered that the inner bead is primarilyresponsible for maintaining burst strength and the outer bead isprimarily responsible for peel strength, a larger inner bead and smallerouter bead result in a good balance between burst strength and ease ofopening.

In one embodiment, the present invention provides a sealed compositecontainer for products having a tubular body member that includes atleast one paperboard body ply. A liner ply is adhered to the innersurface of the tubular body member and includes a barrier layer and aseal layer. The seal layer defines the innermost surface of the linerply and comprises a heat sealable composition. At least one end of thebody member and liner ply is rolled outwardly to form a curled orbead-shaped rim exposing the seal layer. A lid for closing the end ofthe tubular body member is also provided. The lid has a barrier layerand a seal layer adjacent to the seal layer of the liner ply on the rim.The seal layer of the lid also comprises a heat sealable composition.The two seal layers are adhered together to form a heat seal between thelid and the liner ply. The heat seal comprises an inner bead formed ofthe heat sealable composition of the seal layers and facing the interiorof the container and an outer bead also formed of the heat sealablecompositions on the opposite side of the heat seal. The inner bead islarger than the outer bead, meaning that the inner bead comprises alarger amount of the heat seal compositions than the outer bead.

Specifically, the width of the inner bead is about 90 to about 190microns, preferably about 130 to about 180 microns, and the width of theouter bead is about 60 to about 140 microns, preferably about 80 toabout 120 microns. In terms of percentage, the width of the inner beadis about 10 to about 40% greater than the width of the outer bead,preferably about 20 to 30% greater. The heat seal further comprises anintermediate region between the two beads having a width of about 0 toabout 30 microns.

The seal layer of the liner ply is preferably selected from the groupconsisting of high-density polyethylene, low-density polyethylene,metallocene catalyzed polyolefins, and mixtures thereof. The seal layerof the lid is preferably selected from the group consisting of ethylenevinyl acetate, high-density polyethylene, low-density polyethylene,ethylene methyl acrylate, metallocene catalyzed polyolefins, andmixtures thereof.

In one embodiment, the seal layer of the lid comprises about 10lbs./3000 ft² to about 50 lbs./3000 ft² of the heat sealablecomposition, preferably at least about 20 lbs./3000 ft². Additionally,the seal layer of the lid has a thickness of about 0.6 to about 3.3mils.

In the embodiment described above, both the liner ply and the lidinclude separate seal layers. However, the present invention does notrequire the use of two separate seal layers. At least one of said lidand said liner ply must have a seal layer comprising a heat sealablecomposition that forms a heat seal between the lid and the liner ply,but it is not necessary for both the liner ply and the lid to have seallayers.

In one embodiment, the rim of the container defines a heat seal surface,wherein the heat seal surface has an apex portion, an inner portionsloping away from the apex portion and toward the interior of thecontainer, and an outer portion sloping away from the apex portion andtoward the exterior of the container, the inner portion sloping awayform the apex portion at a greater rate than the outer portion. This rimdesign causes the inner bead to extend further along the heat sealsurface toward the interior of the container than the outer bead extendsalong said heat seal surface toward the exterior of the container.

A method of manufacturing tubular containers is also provided. Themethod includes providing a tubular member comprising at least onepaperboard body ply having an inner surface and a liner ply adhered tothe inner surface of the body ply, wherein the liner ply comprises abarrier layer and a seal layer. The seal layer defines the innermostsurface of the liner ply and comprises a heat sealable composition. Atleast one end of the tubular container is rolled outwardly to form a rimexposing the seal layer of the liner ply. A lid is also provided closingthe end of the tubular container, wherein the lid comprises a barrierlayer and a seal layer. The seal layer of the lid comprises a heatsealable composition. The two seal layers are contacted together andheated under conditions that are sufficient to render the heat sealablecompositions of the seal layers flowable. The seal layers are alsopressed together to preferentially encourage flow of the heat sealablecompositions in the direction of the interior of the container to forman inner bead and an outer bead of the heat sealable compositions,wherein the inner bead comprises a larger amount of the heat sealablecompositions than the outer bead. In this manner, the lid ishermetically sealed to the liner ply.

Preferably, the pressing step comprises pressing the seal layerstogether with an inclined surface to preferentially encourage flow inthe direction of the interior container. The heating step preferablycomprises heating the seal layers to about 175° C. to about 275° C. Thepressing step preferably comprises pressing the seal layers together forabout 0.5 to about 1.75 seconds at a sealing pressure of about 30 toabout 60 psi.

As described above, only one seal layer is required. If only one seallayer is used, the rim and lid are placed into contact and the seallayer is heated in order to render the heat sealable composition of theseal layer flowable. The rim and lid are pressed together topreferentially encourage flow in the direction of the interior of thecontainer as described above.

An apparatus for sealing a lid to a container is also provided. Theapparatus includes a sealing head having a central axis and a sealingsurface, wherein at least a portion of the sealing surface is inclinedtowards the central axis of the sealing head. The sealing head has anengaged sealing position and a disengaged position. An actuatoroperatively connected to the sealing head moves the sealing head betweenthe two positions. Preferably, the sealing surface is inclined at anangle from about 2° to about 20°, and most preferably at an angle ofabout 7° to about 12°.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the objects and advantages of the present invention having beenstated, others will appear as the description proceeds when taken inconjunction with the accompanying drawings, which are not necessarilydrawn to scale, wherein;

FIG. 1 is a fragmentary perspective view of a container of the presentinvention illustrating the opening mechanism whereby the liner ply isnot torn through to the underlying paperboard;

FIG. 2 is a fragmentary and enlarged view of the sealed end of thetubular container of an embodiment of the present invention illustratinginner and outer beads of heat-sealable composition;

FIG. 3A is a greatly enlarged sectional view of the present inventiontaken along lines 3A—3A of FIG. 1;

FIG. 3B is a greatly enlarged sectional view of the heat seal of thepresent invention illustrating the initiation of a tear in the seallayers during the opening of the container;

FIG. 3C is a greatly enlarged sectional view of the heat seal of thepresent invention after the container has been opened;

FIG. 4 is a fragmentary and enlarged view of the sealed end of thetubular container of an embodiment of the present invention illustratinga preferred shape of the rim of the container;

FIG. 5A is an enlarged sectional view of the heat-sealing apparatus ofthe present invention prior to contact with the container of the presentinvention;

FIG. 5B is an enlarged sectional view of one embodiment of the sealinghead of the heat sealing apparatus of the present invention contactingthe lid of the container and forcing the heat sealable compositionspreferentially towards the interior of the container;

FIG. 5C is an enlarged sectional view of the sealed end of a containerof the present invention after the sealing operation; and

FIG. 6 is a plan view of an embodiment of an apparatus for making atubular container according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

A tubular container 10 according to the present invention is illustratedin FIG. 1. Although illustrated as having a circular cross section, thetube may have any cross sectional shape, which can be formed by wrappingthe tube around an appropriately shaped mandrel. One example is agenerally rectangular shaped tube having rounded corners.

The embodiment illustrated in FIG. 1 is particularly advantageous forpackaging potato crisps and includes a flexible closure or lid 11, alsoreferred to as a membrane-type closure or lid, and a reusable plasticend cap 12 over the seal. Various other end closures may be used;however, depending upon the type of food product that is to be packagedsuch as, for example, dough.

As illustrated in more detail in FIG. 2, the tubular container 10includes a wall having a body ply 13 which is preferably formed ofpaperboard and a liner ply 14 which is preferably formed of a polymericmaterial adhered to the inner surface of the body ply 13. The upper endof the tubular container 10 is rolled over so as to form a bead-shapedrim 15. The lid 11 is hermetically sealed to the top of the rim 15 asdiscussed below. The end cap 12 is then snapped over the rim 15 and maybe reused after the lid 11 has been removed. A closure (notillustrated), for example a metal closure, can be secured to theopposite end of the container 10.

The lid 11 is constructed of multiple layers. Optionally, the layerdisposed on the outermost surface of the lid 11 away from the inside ofthe tubular container 10 is a paper or paperboard layer 18, such as akraft paper layer. A barrier layer 20 is also provided that serves as abarrier to the passage of liquids and/or gasses such as oxygen. If abarrier is required for both liquids and gasses, the barrier material ispreferably selected from the group consisting of polyethyleneterephthalate, modified polyethylene terephthalate, polyethylenenapthalate, metallized polyester or metallized polypropylene andmixtures thereof. The barrier layer 20 may also be constructed of metaloxide and silicate coated polyester, metal oxide and silicate coatedpolypropylene, ethylene vinyl alcohol and mixtures thereof.Alternatively, the barrier layer 20 comprises an aluminum foil layer.

Advantageously, the lid 11 further includes a seal layer 22 comprising aheat sealable composition and positioned such that the seal layer 22 ofthe lid 11 is adjacent to the seal layer 26 of the liner ply 14. Theseal layer 22 of the lid 11 is preferably constructed of a materialselected from the group consisting of ethylene vinyl acetate, highdensity polyethylene, low density polyethylene, ethylene methylacrylate, metallocene catalyzed polyolefins and mixtures or blendsthereof. The seal layer 22 of the lid 11 preferably has a melting pointwithin the range of about 70° C. and 130° C. Most preferably, themelting point of the seal layer 22 is between about 80° C. and 110° C.and in particular is 83° C.

In one embodiment, the lid 11 is formed as a laminate having apaperboard layer 18 adhered to the barrier layer 20 using a coextrudedadhesive layer (not shown). The adhesive layer is constructed ofmaterials selected from the group consisting of low densitypolyethylene, ethylene methyl acrylate (EMA), ethylene-methacrylic acidcopolymers (EMAA) and mixtures thereof. The seal layer 22 is coated onthe opposing surface of the barrier layer 20. The seal layer 22 may beformed by extrusion coating, as a blown film laminated by extrusion oras a blown film laminated with a thermoset adhesive. In one embodiment,the seal layer 22 is formed as a dual layer coextrusion of high densitypolyethylene and ethylene methacrylate copolymer.

The seal layer 22 of the lid 11 is preferably between about 0.6 andabout 3.0 mils in thickness, most preferably at least about 1.5 mils inthickness. The seal layer 22 comprises a heat sealable compositionweight between about 10 to about 50 lbs./3000 ft² and preferably about20 to about 40 lbs./3000 ft². Most preferably, the seal layer 22 has aheat sealable composition weight of about 25 lbs./3000 ft² or more. Therelatively thicker seal layer 22 prevents natural variations in thecontainer manufacturing process from affecting the consistency of theheat seal. For example, imperfections in the rim 15 and variations inthe container height have a significant effect on the sealing process.The additional heat seal material fills any cracks and fissures createdin the rim 15 and is also able to create a continuous seal around seamsin the container wall, such as the seams created by anaconda folds oroverlap seams in the liner. The additional seal material alsocontributes to better sealing by compensating for slight differences incontainer height that might otherwise lead to a reduction in sealstrength. Further, the increased amounts of seal layer 22 material allowcreation of a heat seal despite the presence of contaminants introducedinto the heat seal area during the manufacturing process, such as wax.By using a thicker seal layer, the heat seal may be formed at lowersealing temperatures. A preferred construction of the seal layer isdisclosed in U.S. patent application Ser. No. 09/416,194, now U.S. Pat.No. 6,302,321, filed concurrently herewith and entitled “Sealant Layerfor Container Lid.” This application is assigned to the assignee of thepresent application and is expressly incorporated herein by reference.

The liner ply 14 is also typically constructed of multiple layers. Withthe exception of the outermost seal layer 26, the composition of theliner ply 14 is not critical to the present invention. Preferably, oneof the layers forms a barrier to moisture and/or gasses, depending onthe application. It will be understood that various barrier materialsand liner plies could be employed depending upon the item beingpackaged. For example, conventional liners include a layer of foilbacked with kraft paper. However, in a preferred embodiment, the linerply 14 is substantially entirely formed of polymeric material. Inparticular, liner plies such as described in U.S. Pat. No. 5,829,669 toDrummond et al. or U.S. Pat. No. 5,846,619 to Cahill et al. both ofwhich are assigned to the assignee of the present invention and arehereby incorporated by reference, may be used.

In the embodiment illustrated in FIG. 2, the liner ply 14 includes aseal layer 26, a moisture barrier layer 28 and an adhesive layer 30. Thebarrier layer 28 is resistant to the passage of liquids and gasses suchas oxygen. If a high barrier is required for both liquids and gasses,preferred barrier materials are metallized polyester or metallizedpolypropylene. Some food products, such as juices, do not require a gasbarrier and other barrier materials may be used (although the barriermay also be generally resistant to the passage of gasses). It will beunderstood that various barrier materials could be employed dependingupon the item being packaged. Alternative barrier materials includenylon, EVOH (ethylene vinyl alcohol polymer and copolymer),polyvinylidene chloride, polyethylene, polypropylene, metallizedpolypropylene, metal oxide and silicate coated polyester, metal oxideand silicate coated polypropylene and the like as will be apparent tothe skilled artisan.

One surface of the barrier layer 28 may include a thin metallizedcoating 32 to provide a metallic appearance and also to enhance thebarrier properties. The metallized coating 32, which may be formed ofaluminum, is significantly thinner than a foil layer, however, and isnot necessary for strength or barrier properties in certainapplications.

An adhesive layer 30 is preferably below the metallized coating 32 anddefines the radially outermost surface of the liner ply 14. The adhesivelayer 30 may have multiple layers coextruded together. The adhesivelayer 30 may be selected from the group consisting of metallocenecatalyzed polyolefins, ethylene-methacrylic acid, ethylene methylacrylate, ethylene butyl acrylate, ethylene acrylic acid, ethylene vinylacetate, and blends, mixtures and copolymers thereof. The adhesive layer30 may also be a thermoset adhesive layer.

A seal layer 26 defines the radially innermost surface of the liner ply14. The seal layer 26 provides a surface against which the adhesivelayer 30 is adhered when a first marginal edge portion 41 of the linerply 14 is brought into an overlapping relationship with a secondmarginal edge portion 42, as shown in FIG. 6. The seal layer 26 alsoforms the heat seal between the lid 11 and the liner 14 in conjunctionwith the seal layer 22 of the lid.

The seal layer 26 of the liner ply 14 is preferably constructed of amaterial selected from the group consisting of high densitypolyethylene, low density polyethylene, metallocene catalyzedpolyolefins and mixtures or blends thereof. In embodiments of the seallayer 26 including a polyolefin polymer, the polyolefin is preferablyhigh density polyethylene or a high density polyethylene blendcontaining up to 30% low density polyethylene. The seal layer 26 of theliner ply 14 preferably has a melting point within the range of about110° C. and about 140° C. Most preferably, the seal layer 26 has amelting point between about 120° C. and 130° C.

FIG. 2 illustrates the sealed end of the tubular container of apreferred embodiment of the present invention wherein the two seallayers, 22, 26 are heat sealed together. As shown in greater detail inFIG. 3A, a sealed composite container for products is provided having aheat seal between the liner ply 14 and the lid 11 in the form of aninner heat seal bead 36 and an outer heat seal bead 38. The inner heatseal bead 36 and the outer heat seal bead 38 are formed of the heatsealable compositions of the seal layer 26 of the liner ply 14 and theseal layer 22 of the lid 11. The heat sealable compositions of both seallayers 22, 26 are displaced outwardly from the intermediate regionduring the heat sealing operation and are cooled to form the beads 36,38. The inner heat seal bead 36 faces the interior of the tubularcontainer 10 and the outer heat seal bead 38 is disposed on the oppositeside of the heat seal area from the inner heat seal bead 36. Whencooled, the heat seal comprises a thin intermediate region 40 betweenthe inner heat seal bead 36 and the outer heat seal bead 38. In certainplaces, the heat sealable compositions may be completely displaced frombetween the barrier layers 20 and 28 such that the barrier layers are inabutting contact. However, the inner and outer beads 36, 38 maintaindouble barriers against the passage of liquids and gasses so that ahermetic seal is maintained. The intermediate region 40 preferably has alower bond strength than the inner heat seal bead 36 and the outer heatseal bead 38. The width of the intermediate region 40 is about 0 toabout 30 microns. The term “bead” as used herein is intended to bedistinguished from prior containers having relatively flat heat sealwhere very little, if any, flowing of the heat seal compositions occurs.In addition, this embodiment is not limited to use with only linershaving a straight overlapping seam, but the heat seal beads 36, 38 couldalso be used with an anaconda fold seam.

In effect, the inner heat seal bead 36 and the outer heat seal bead 38provide a double seal having a high tensile or burst strength. The burststrength of the bead seals gives the container 10 a strong seal againstforces acting upon the container in a direction normal to the heat seal(i.e., normal to the plane defined by the end of the tubular container10). Since most forces acting upon a container during storage andtransit will occur normal to the heat seal area, the high burst strengthof the inner heat seal bead 36 and outer heat seal bead 38 of thepresent invention is especially advantageous for use with productcontainers. Burst strength may be tested using an altitude chamber.Typically, the sealed container 10 is placed in the altitude chamber andthen subjected to an external partial vacuum for a predetermined periodof time to determine whether the heat seal is capable of withstandingdifferences between interior container pressure and external airpressure. Suitable testing conditions include subjecting the containerto a vacuum of 10 in. of Hg for 30 minutes at room temperature. Thecontainers 10 of the present invention are capable of maintaining ahermetic seal during a thirty minute exposure to a vacuum of 10 in. ofHg at room temperature.

Notwithstanding the high burst strength, the peel strength of the heatseal formed according to the present invention is relatively low,resulting in a container that exhibits relative ease of opening. This isin contrast to conventional containers where two SURLYN® polymers areionically crosslinked together in a relatively flat heat seal, resultingin a bond, which is sufficient to tear the liner 14 when removing thelid 11 (rather than tearing through the SURLYN® polymer). The preferredrange for peel strength is about 5 to about 10 lbs./linear inch. In oneembodiment, the heat seal has a peel strength of about 7 to about 10lbs./linear inch. Thus, the heat seal of the present invention combinesthe shear strength and tensile strength necessary to prevent unwantedbreaches of the tubular container 10 with relatively low peel strengthfor ease of opening by the consumer.

FIGS. 3A-3C illustrate the opening mechanism for container 10 of thepresent invention. As shown by FIGS. 3B and 3C, the shearing forcegenerated during opening of the container 10 causes a tear to propagatethrough the seal layer 26 of the liner ply 14 and/or the seal layer 22of the lid 11. The seal layer 26 of the liner ply 14 and the seal layer22 of the lid 11 provide a bond strength between the barrier layer 20 ofthe lid 11 and the barrier layer 28 of the liner ply 14 that is lowerthan the bond strength between the barrier layer 28 of the liner ply 14and the paperboard body ply 13 (or any other intermediate layers such asthe metallized coating 32 of the liner 14). As a result, shearing thattakes place during the opening of the tubular container 10 occurs onlybetween, and not through, the barrier layers of the liner ply 14 and lid11. When the tubular container 10 of the present invention is opened,unsightly tears through the barrier layer 28 of the liner ply 14 do notoccur.

It has been discovered that the inner bead 36 of the double bead sealprovides the primary resistance to tensile forces acting upon thecontainer, such as those burst forces generated by changes in internalpressure during transport. However, the outer bead 38 provides theprimary resistance to opening by peeling of the peelable heat sealformed between the lid 11 and the liner ply 14. As a result, it has beendiscovered that the heat seal is advantageously formed having a largerinner bead 36 and a smaller outer bead 38. The resulting containerexhibits both improved ease of opening by virtue of the smaller outerbead 38 and improved burst strength for withstanding the rigors oftransportation by virtue of the larger inner bead 36.

The heat seal of the present invention has an inner bead 36 having agreater width than the outer bead 38. Bead width is defined as thedistance between the barrier layer 20 of the lid 11 and the barrierlayer 28 of the liner ply 14 measured at the longest point across thebead in the vertical plane as shown by reference symbol A of FIG. 3A.Note that the width measurement is generically defined as the distancebetween the next adjacent layer to the seal layer 22 of the lid 11 inthe construction of the lid and the next adjacent layer to the seallayer 26 of the liner 14 in the construction of the liner. In thepreferred embodiment, the next adjacent layers are the barrier layers.However, in other embodiments, the next adjacent layers may be layers ofother types. Specifically, the width of the inner bead 36 is about 90 toabout 190 microns, preferably about 130 to about 180 microns, and mostpreferably about 140 to about 160 microns. The width of the outer bead38 is about 60 to about 140 microns, preferably about 80 to about 120microns, and most preferably about 95 to about 120 microns. As will beunderstood, the width of the beads will depend on a number of factors,including the heat sealable materials used to form the seal layers 22,26, the heat sealing conditions and the like.

In terms of relative widths of the inner bead 36 and outer bead 38, thewidth of the inner bead is about 10 to about 40% greater than the widthof the outer bead, preferably about 20 to 30% greater, and mostpreferably about 22 to about 26% greater.

A preferred shape of the rim 15 of the container 10 is illustrated inFIG. 4. As shown, the rim 15 of the container 10 is rolled outwardly toexpose the heat seal layer 26 of the liner 14. The rim 15 creates a heatseal surface that contacts the seal layer 22 of the lid 11. The heatseal surface includes an apex portion 84, an inner portion 86 slopingaway from the apex portion and toward the interior of the container 10,and an outer portion 88 sloping away from the apex portion and towardthe exterior of the container. Preferably, the inner portion 86 slopesaway from the apex portion 84 at a greater rate than the outer portion88. This rim 15 design encourages the inner bead 36 to extend furtherdown the heat seal surface toward the interior of the container 10 anddiscourages the outer bead 38 from extending down the heat seal surfacetoward the exterior of the container. In this manner, the shape of therim 15 affects the direction of flow of the heat sealable compositionsand the relative shapes of the inner bead 36 and outer bead 38. Byencouraging flow of the heat sealable compositions further down the heatseal surface toward the interior of the container, better burst strengthis obtained due to the anchoring effect of the inner bead 36. It isbelieved that the shape of the inner bead 36 creates a shear componentof resistance which counters the forces caused by internal containerpressure more effectively than the more tensile resistance of prior artdesigns. Additionally, peel strength is maintained at a reasonable levelbecause the incline or slope of the heat seal surface toward theexterior of the can is lesser in degree and does not encourage flow ofthe heat sealable compositions in that direction. As desired, theabove-described shape of the rim 15 maintains the outer bead 38 at arelatively smaller size as compared to the inner bead 36. The desiredrim 15 shape can be formed during the initial formation of the containerrim or by a secondary forming process. Alternatively, the desired rim 15shape may be formed in conjunction with the heat sealing operation. Therim can also have a substantially planar portion as shown in FIG. 4 anddisclosed in U.S. patent application Ser. No. 09/416,194, now U.S. Pat.No. 6,302,321, filed concurrently herewith and entitled “Container WithHeat Seal Surface Having a Substantially Planar Portion.” Thisapplication is assigned to the assignee of the present application andis expressly incorporated herein by reference.

A method and apparatus for sealing a container for products is alsoprovided. The sealing method and apparatus are illustrated in FIGS.5A-5C. The heat seal may be created using any suitable apparatus knownin the art. In some sealing systems, wax is applied to the rim 15 tohold the lid 11 in place prior to formation of the heat seal. As notedabove, the relatively thick seal layer 22 of the lid 11 allows theformation of a heat seal having sufficient bond strength despite thepresence of wax in the heat seal area. The present invention is alsocompatible with sealing systems that utilize a vacuum system toinitially hold the lid 11 in place prior to the heat sealing operation.

Using either apparatus, the heat sealing method of the present inventionincludes providing a tubular member having a paperboard layer 13 andliner ply 14 adhered to the inner surface of the paperboard layer. Asdescribed above, the liner ply 14 includes a barrier layer 28 and a seallayer 26, the seal layer defining the innermost surface of the liner plyand comprising a heat sealable composition. Once an end of the tubularmember is rolled outwardly to form a rim 15, a lid 11 may be contactedwith the liner ply 14 for forming the heat seal. The lid 11 includes abarrier layer 20 and a seal layer 22, wherein the seal layer comprises aheat sealable composition. The seal layer 22 of the lid 11 is contactedwith the seal layer 26 of the liner ply 14. The two seal layers 22, 26are then heated under conditions sufficient to render the heat sealablecompositions flowable and pressed together so as to preferentiallyencourage more flow of the heat sealable compositions in the directionof the interior of the container to form an inner bead 36 and an outerbead 38, wherein the inner bead contains a larger amount of heatsealable compositions than the outer bead.

In a preferred embodiment, the pressing step is accomplished by pressingthe seal layers 22, 26 together using an inclined surface, such asinclined heat sealing head 44. The heat sealing head 44 is preferablyconstructed of metal, such as aluminum, coated copper or other heatconductive material. The heat sealing head 44 is heated by heat source46. The heat source may be any suitable type of heat source known in theart. The heat sealing head 44 does not have to be heated. The heat seallayers 22, 26 could be heated independently using a separate heatsource. The heat sealing head 44 has an engaged sealing position incontact with the lid 11 and a disengaged position. The heat sealing head44 is moved between the two positions by an actuator 48. The actuator 48may be any type of actuator known in the art, including mechanical,pneumatic, and the like.

The angle of the inclined surface of the heat sealing head 44 affectsthe amount of material that flows to form the beads as well as therelative size of the beads. The angle of the inclined surface of theheat sealing head 44 is about 2 to about 20 degrees, preferably about 7to about 12 degrees. In one embodiment, the angle of the inclinedsurface is about 10 degrees. In another embodiment, the angle is about3°. The inclined surface of the head 44 causes molten polymer from theseal layers to move towards the interior of the container to form theinner bead. As this movement occurs, the molten polymer advantageously“fills in” any irregularities in the liner and lid surfaces, thusimproving the integrity of the seal.

The heat sealing conditions, such as temperature, pressure, and time,depend on a number of factors, including the heat sealable compositionsused and the thickness of the heat seal layers. In one embodiment, theheat seal layers are heated to between about 175° C. to about 275° C.,preferably about 205° C. to about 230° C., and most preferably about210° C. to about 225° C. In one embodiment, the heat sealing temperatureis about 218° C. The heat sealing pressure is about 30 to about 60 psi,preferably about 40 to about 50 psi. In one embodiment, the heat sealingpressure is about 45 psi. The heat sealing time, meaning the period oftime during which heat sealing pressure is applied, is about 0.5 toabout 1.75 seconds, preferably about 0.9 to about 1.5 seconds, and mostpreferably about 1.15 to about 1.35 seconds. In one embodiment, the heatsealing time is about 1.25 seconds.

Although the container embodiments discussed above include two seallayers, 22 and 26, the present invention does not require the use of twoseal layers. At least one of the liner and lid must include a seal layerin order to provide the necessary heat seal beads, 36 and 38, asdescribed above. However, two seal layers are not necessary to practicethe present invention. If a single heat seal layer is used, the heatseal layer may be constructed of high density polyethylene, low densitypolyethylene, ethylene vinyl acetate, ethylene methyl acrylate,metallocene catalyzed polyolefins and mixtures thereof.

The containers 10 of the present invention may be manufactured by theprocess illustrated in FIG. 6. As shown, a continuous strip ofpaperboard body ply material 13 is supplied to the apparatus and isfirst passed through a pair of opposed edge skivers 50. The edge skiversremove part of the square edge of the body ply 13 to create first 52 andsecond 54 edges having a beveled configuration. The body ply 13 is thenadvanced through an adhesive applicator 56, which applies an adhesive 21to the upper surface of the body ply 13. The adhesive 21 isadvantageously an aqueous adhesive, which overcomes the many problemsassociated with solvent based adhesives. No special equipment is neededto capture solvents, which evaporate from the adhesive in order tocomply with environmental regulations. Preferred adhesives are aqueouslow glass transition temperature ethylene vinyl acetate (>18%)materials. One preferred adhesive is No. 72-4172, which is availablefrom the National Starch and Chemical Company. Another adhesive that maybe used is No. 33-4060, which is also available from the National Starchand Chemical Company. The adhesive 21, as well as other adhesive layersused to construct the container 10, may be applied in the form of a foamas described in copending U.S. patent application Ser. No. 09/197,275entitled, “Composite Container Having Foamed Adhesive,” which isassigned to the assignee of the present invention and herebyincorporated by reference.

The body ply 13 and wet adhesive 21 applied thereto are then passedunderneath a heater 58 which evaporates at least part of the watercontent of the aqueous adhesive 21 to render the adhesive substantiallytacky. It is important that the correct amount of heat is supplied tothe adhesive. Insufficient heat will not evaporate enough water in asufficiently short period of time with the result that the adhesive willnot be rendered sufficiently tacky. Conversely, too much heat willoverdry the adhesive and cause the adhesive to lose tackiness. Apreferred type of heat source is an infrared heater although variousother heat sources, e.g., forced air heating or the like can be used.After heating the adhesive 21 on the body ply 13, the body ply 13 andthe liner ply 14 are fed to the shaping mandrel from oppositedirections. The body ply 13 is passed under skive adhesive applicator 60which applies the skive adhesive 24 to the beveled surface of the skivedsecond edge 54 of the body ply 13. The skive adhesive 24 is preferably ahot melt adhesive of the type which is conventional in the art, althoughit could also be a water based adhesive including one or more polymers.Polyvinyl acetate and ethylene vinyl acetate are the preferred liquidadhesives. The skive adhesive 24 helps provide a stronger body ply bondespecially for single body ply containers.

The surface of the liner ply 14 that contacts the body ply 13 issubjected to a corona treatment station 62. The opposite surface ofliner ply 14 is coated with lubricant from a roller 64, which allows theliner ply to slide smoothly during the winding operation.

The liner ply 14 is then passed under an infrared heater 66, which heatsthe second marginal edge portion 42 of the liner ply. After the infraredheater 66, the second marginal edge portion 42 of the liner ply 14 isthen passed under at least one forced air heater 68.

The body ply 13 and the liner ply 14 are then wrapped around a shapingmandrel 70 from opposite sides of the mandrel. Each ply is first wrappedunder the mandrel 70 and then back over the top in a helical fashionwith the liner ply 14 wound against the surface of the mandrel. Thefirst marginal edge portion 41 of the liner ply 14 is exposed on themandrel 70 and is subjected to heat from a second forced air heater 72.

As the body ply 13 is further wrapped and the first edge 52 of the bodyply 13 advances back under the mandrel 70 after one complete revolution,it is brought into contact with the second edge 54 of the ensuingportion of the body ply 13 which is first coming into contact with themandrel. The skived edges 52, 54 become abutted together and the skiveadhesive 24 adheres the edges together to form a spirally wound tubewhich advances along the mandrel 70.

With regard to the liner ply 14, the first marginal edge portion 41 isbrought into an overlapping relationship with the second marginal edgeportion 42 to create a sealed straight lap seam. The seal is formed by apolymeric adhesive layer 30 of the first marginal edge 41 becomingbonded to the second marginal edge 42. However, a strip of hot meltadhesive could alternatively be used for securing and sealing the lineroverlap.

The tube is then advanced down the mandrel 70 by a conventional windingbelt 74, which extends around a pair of opposed pulleys 76. The windingbelt 74 not only rotates and advances the tube, but also appliespressure to the overlapping edges of the body ply 13 and liner ply 14 toensure a secure bond between the respective ply edges.

An outer label ply 16 is then preferably passed over an adhesiveapplicator 78 and wrapped around the body ply 13. The label ply 16 couldbe applied before the winding belt 74. At a cutting station 80, thecontinuous tube is cut into discrete lengths and removed from themandrel 70.

The ends of the containers 10 are then rolled outwardly to form the rim15 and the lid 11 is subsequently heat sealed thereto as describedabove. An end closure, such as a metal closure, is attached to the otherend of the container 10. Typically, the lid 11 and end closure 12 areapplied to one end of the container 10 prior to filling of thecontainer. After filling, an end closure is applied to the opposing end.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. For example, the tubularcontainers according to the present invention are not necessarilyhelically wound but may instead be longitudinally wrapped to create a“convolute” tube having an axially extending seam. In addition, althoughthe tubular containers according to the present invention have beendescribed primarily in connection with food products, it is to beunderstood that the containers could be used in connection with otherproducts where the liner ply is advantageous such as, for example, inkor caulk. Although specific terms are employed herein, they are used ina generic and descriptive sense only and not for purposes of limitation.

1. A method of manufacturing a sealed container for products comprising:providing a tubular member comprising at least one paperboard body plyhaving an inner surface and a liner ply adhered to the inner surface ofthe body ply, the liner ply comprising a barrier layer; rollingoutwardly at least one end of the tubular member to form a rim;providing a lid for closing the end of said tubular member, the lidcomprising a barrier layer, at least one of said lid and said liner plyfurther comprising at least one seal layer comprising a heat solublecomposition, the seal layer being operatively positioned to form a heatseal between the lid and the liner ply; contacting the rim and the lid;heating the seal layer under conditions sufficient to render the heatsealable composition of the seal layer flowable; pressing the rim andthe lid together to preferentially encourage flow of the heat sealablecomposition in the direction of the interior of the container to form aninner bead and an outer bead of the heat sealable composition, the innerbead comprising a larger amount of the heat sealable composition incross-section than the outer bead, thereby hermetically sealing the lidto the liner ply, wherein the heat seal comprising the inner bead andthe outer bead extends substantially continuously around the entirecircumference of the rim.
 2. A method according to claim 1, wherein saidpressing step comprises pressing the rim and lid together with aninclined surface to preferentially encourage flow of the heat sealablecomposition in the direction of the interior of the container.
 3. Amethod according to claim 1, wherein said heating step comprises heatingthe seal layer to about 175° C. to about 275° C.
 4. A method accordingto claim 1, wherein said pressing step comprises pressing the rim andlid together for about 0.5 to about 1.75 seconds.
 5. A method accordingto claim 1, wherein said pressing step comprises pressing the rim andlid together at a pressure of about 30 to about 60 psi.
 6. A methodaccording to claim 1, wherein said rolling step comprises rollingoutwardly at least one end of the tubular member to form a rim such thatthe rim defines a heat seal surface, said heat seal surface having anapex portion, an inner portion sloping away from said apex portion andtoward the interior of the container, and an outer portion sloping awayfrom said apex portion and toward the exterior of the container, theinner portion sloping away from the apex portion at a greater rate thanthe outer portion.
 7. A method according to claim 1, wherein the widthof the inner bead is about 90 to about 190 microns and the width of theouter bead is about 60 to about 140 microns.
 8. A method according toclaim 7, wherein the width of the inner bead is about 130 to about 180microns and the width of the outer bead is about 80 to about 120microns.
 9. A method according to claim 1, wherein the width of saidinner bead is about 10 to about 40% greater than the width of said outerbead.
 10. A method according to claim 9, wherein the width of said innerbead is about 20 to about 30% greater than the width of said outer bead.11. A method according to claim 1, wherein the seal layer is selectedfrom the group consisting of high density polyethylene, low densitypolyethylene, ethylene vinyl acetate, ethylene methyl acrylate,metallocenes and mixtures thereof.
 12. A method according to claim 1,wherein the barrier layer of the lid is selected from the groupconsisting of polyethylene terephthalate, modified polyethyleneterephthalate, metallized polyethylene terephthalate, metallizedmodified polyethylene terephthalate, polyethylene napthalate, metallizedpolyethylene napthalate, metallized polypropylene, metal oxide andsilicate coated polyester, metal oxide and silicate coatedpolypropylene, ethylene vinyl alcohol and mixtures thereof.
 13. A methodaccording to claim 1, wherein the barrier layer or the liner ply isselected from the group consisting of polyethylene terephthalate,modified polyethylene terephthalate, metallized polyethyleneterephthalate, metallized modified polyethylene terephthalate,polyethylene napthalate, metallized polyethylene napthalate, metallizedpolypropylene, metal oxide and silicate coated polyester, metal oxideand silicate coated polypropylene, ethylene vinyl alcohol and mixturesthereof.
 14. A method according to claim 1, wherein the heat sealcomprising the inner bead and the outer bead maintains a hermetic sealduring a thirty minute exposure to a vacuum of 10 inches of Hg at roomtemperature.
 15. A method according to claim 1, wherein the lidcomprises a seal layer having a thickness of about 0.6 to about 3.3mils.
 16. A method according to claim 15, wherein the seal layer of thelid has a thickness of at least about 1.5 mils.
 17. A method accordingto claim 1, wherein both the lid and the liner ply comprise a seallayer.
 18. A method according to claim 17, wherein the seal layer of thelid has a lower melting point than the seal layer of the liner ply. 19.A method according to claim 18, wherein the melting point of the seallayer of the lid is between about 70° C. to about 130° C. and themelting point of the seal layer of the liner is between about 110° C. toabout 140° C.
 20. A method according to claim 19, wherein the meltingpoint of the seal layer of the lid is between about 80° C. to about 110°C. and the melting point of the seal layer of the liner is between about120° C. to about 130° C.
 21. A method according to claim 1, wherein theheat seal provides a bond strength between the barrier layer of the lidand the barrier layer of the liner ply, the bond strength being leasthan the bond strength between the barrier layer of the liner ply andthe tubular body member behind the barrier layer such that, uponopening, shearing occurs only between the barrier layers.